There are no mirrors in the Tower of Ghouls that I can see. You might mean chairs. To open the northwest gate there, you need an Autographed Amulet. Someone gives it as a quest prize i think but you can also buy one from Epona the Jeweler if you sleep outside her shop long enough. The other gate there in the middle of the north, you need a Gold Figurine that's on a shelf there.

Hi Cat,
I’m getting close to finishing this expansion, which has been another brilliant addition to the game. I too am struggling to open the gate opposite the main entrance. I see you’ve mentioned a secret switch but I can’t find that either. Please can you help?
Many thanks 🙂

Help! Northwest gate, I have the Autographed Amulet, I've spoken with the gatekeeper, and I've answered all the chair questions... Gate is still locked. What am I missing? (...Not sure if it matters, I've reached Sapho and got the poetry book too. )